1. Field of the Invention
The present invention relates to a head-mounted display apparatus, and more particularly, to a head-mounted display apparatus which displays images in a manner such that they can be observed while wearing the apparatus on one's head.
2. Description of the Related Art
Hitherto, display apparatuses which are mounted to heads and by which images are observed, such as HMD (Head-Mounted Display) and HUD (Head-Up Display) have been known.
This type of display apparatuses are described, for example, in Japanese Unexamined Patent Application Publication No. 2003-248194. In the image display apparatus described in the Publication, an image disposed on a spectacle frame is projected to a hologram optical element as a combiner. The image reflected by the hologram optical element is guided to an observer's eye, so that this image as a virtual image is superimposed on the view of the external environment.
On the other hand, Japanese Unexamined Patent Application Publication No. 6-78247 describes a head-mounted display apparatus with which the operation of changing over an electronic image to an external environment image can be easily performed. Also, the Publication describes a technique by which a virtual screen can be observed as if it were stationary at a predetermined position of the external environment, although the tilting angle of an observer's head changes, and moreover, an image of the external environment can be observed on the outside of the virtual screen (see Step Nos. 0035 to 0044, particularly, Paragraph No. 0041 in the Publication).
As regards human eyes, optic cells especially having a high visual acuity, which are called pyramids, are distributed on the center of a retina at a high density, and the distribution-density of the optic cells remarkably decreases toward the periphery of the retina. There are three types of pyramids, of which the spectral sensitivity characteristic peaks exist in the R, G, and B. Thus, the sense of color is effective in the range having a radius of 20° to 30° around the center of the retina. The range having a radius from the center of up to 3° in which the pyramids are distributed at a very high density is called a center orbit. High-sensitive cells, called rods, which are reactive with brightness and are effective mainly in seeing in dim light, are distributed more and more in the direction of from the center toward the periphery, instead of the pyramids. The density of the rods is the highest in the position from the retina-center of about 20°, and is gradually reduced toward the periphery. The visual acuity of a human eye having the above-described structure is varied with brightness. It is known that when the brightness is high, the visual acuity is highest in the center orbit, and remarkably decreases near the periphery. FIG. 34 illustrates the visual acuity at the respective positions on the retina plotting the brightness as a parameter (Ikeda Mitsuo: “Newest Applied Physics (Saishin Oyo-Buturigaku) Series 3”, Psychological Physics of Visual Sense (Shikaku no Shinri-Butsurigaku), p. 194, 1975, published by Morikita Shuppan). As seen in FIG. 34, when the brightness is relatively high, the pyramids distributed at a high density near the center orbit effectively act. Thus, the visual acuity is very high especially in the center of the retina. However, when the brightness is low, the pyramids do not effectively act, so that the difference between the visual acuities in the center and in the periphery is not significant. Then, it is generally known that the two-point discrimination resolving-power of the center orbit is about 1′.
In recent years, from the standpoint of the global environment protection, the saving of consumption energy has been attempted in various types of devices and apparatuses. Specifically, for example, there is known a technique in the personal computer field, by which PCs are automatically set in a low consumption power mode, when no input-operation is carried out through a keyboard, a mouse, or the like for a predetermined time-period.
According to the known HMD and HUD, even if the head is moved to a small degree, the observed virtual view is moved together with the head. Thus, the screen is observed as if it went around the head. Particular, when the observer wears the device for a long time, the screen becomes a factor in stress to the observer.
Moreover, the above-described known display apparatuses have a small size and a light weight, and are satisfactorily portable. Thus, the observer tends to move in an ordinary manner while wearing the apparatus. However, if the display screen continues to be displayed in the field of sight of the observer, the screen may disturb the ordinary movement. In order to eliminate such disturbance, it is necessary to remove the display apparatus or switch off the power supply of the apparatus before the ordinary movement is made. However, in the case of the spectacle as an example, it is troublesome to take off the spectacle prior to each movement. Similarly, it is troublesome to remove the display apparatus or switch off the power supply of the apparatus every time the ordinary movement is made.
In the known display apparatuses, similarly, it is necessary to switch off the power supply for example, for the purpose of reducing the consumption power. However, it is troublesome to frequently switch off the power supply. It is unsuitable to apply the above-described technique for reducing the consumption power used in personal computers, that is, the technique by which the power supply is controlled in the case in which input-operation is not executed for a predetermined time-period, to information display apparatuses for which intentional input-operation is not necessarily carried out. For display apparatuses which use batteries or the like as power supply and are portable, it is very important to reduce the supply power.